Various methods have been proposed for preparing drugs that are capable of maintaining the concentration of the effective ingredient in blood for an extended period of time. In particular, active efforts have been made to prepare orally administered drugs that let the active ingredient be dissolved in a given quantity in digestive tracts. It is possible by the state-of-the-art technology to cause the drug substance to be dissolved at a constant rate, but its absorption rate is decreased as it moves toward the lower part of digestive tracts, so with conventional long lasting drugs for oral administration, there is high possibility that the concentration of the active ingredient in blood is decreased as they move along the digestive tract although they have a constant dissolution rate.
As a result of various studies to solve this problem, the present invention have found the following: (1) if part of a tablet is covered with an inert base having substantially the same disintegration rate, the dissolution of the drug substance is prevented for a certain period after oral administration, and when the tablet reaches a site in the digestive tract where the rate of absorption of the drug substance is decreased, the disintegration of the inert base is completed so as to increase the area of the tablet from which the drug substance is released, hence the amount of the substance to be adsorbed by the digestive tract; (2) the base for making such multiple-layer tablet is preferably a mixture of a water-insoluble wax that is solid at ordinary temperatures, a disintegrator and a binder; and (3) the rate of disintegration of the base can be controlled over an extended period by finely micronizing the water-insoluble wax to an average particle size of 10 .mu.m or less, preferably 5 .mu.m or less, with a jet mill or other suitable means.